AD 496: Toughness testing of bolts

Bolts used in environments subject to low temperatures that fall outside the range of temperatures usually encountered, such as in cold stores, may require toughness testing at the service temperature. SCI has recently been contacted regarding testing bolts that are smaller than the limiting diameter for normal Charpy impact tests. The purpose of this note is to advise how impact testing can be carried out.

AD 332: Toughness of bolts advised that non-preloaded and preloaded bolts supplied to BS EN 15048:2007 and BS EN 14399-3:2005 would be tested at -20°C and have a Charpy V-notch impact strength of at least 27 joules.

Bolt materials comply with BS EN ISO 898-1:2013¹ which indicates in clause 1 Scope, Note 1 that “Fasteners conforming to the requirements of this part of ISO 898 are used in applications ranging from -50°C to + 150°C. …”. Notwithstanding this, clause 9.14.1 indicates that impact tests are carried out only if required by a product standard or agreed between manufacturer and purchaser. The standard indicates tests are to be carried out in accordance with ISO 148-1 (Charpy V-notch test) at -20°C and are required to achieve a Charpy V-notch impact strength of at least 27 joules, for bolts of size M16 or greater (see Table 3). No tests are carried out to demonstrate impact strength at temperatures below -20°C unless specified but the standard indicates that other test temperatures and impact strength values can be called for.

The bolt size limitation appears to be indicated so that standard Charpy test pieces can be produced from the bolts. (M16 bolts have a tensile stress area of 157 mm² and a corresponding diameter of 14.1mm. The diagonal dimension of a 10mm square Charpy test piece is 14.1mm). Tests specified in ISO 898-1 are applicable to machined tests pieces made from bolts, screws and studs of diameter at least 16mm. The total length of the test pieces is at least 55mm.

If impact tests are required on bolts of smaller size than M16, these can also be carried out in accordance with ISO 148-1, which allows for 7.5 mm, 5 mm and 2.5 mm square samples of 55 mm length. Such tests are not strictly in accordance with ISO 898-1 but will allow the impact properties of fasteners of smaller size than M16 to be determined at temperatures required by the purchaser. There is however no published basis for correlating test results from the smaller test pieces with results from standard ones so acceptance criteria should be agreed before supply. Test pieces with the standard length, depth and notch size but reduced width can also be used. The impact energy of such test pieces can be adjusted pro rata with the cross-sectional area at the notch for comparison with standard test requirements and would allow the impact properties of M12 bolts to be determined.

Contact: Richard Henderson
Tel: 01344 636555
Email: advisory@steel-sci.com

1. BS EN ISO 898-1:2013 Mechanical properties of fasteners made of carbon steel and alloy steel Part 1: Bolts, screws and studs with specified property classes – coarse thread and fine pitch thread (ISO 898-1:2013), BSI